Screw



L W. CALDWELL.

' SCREW. APPLICATLQN'FILED DEC- 14, me.

Patented Apr. 13,1920.

- 2 SHEETS-SHEET 1.. W.'.CALDWELLI.

SCREW.

APPLICATIGH FILED DEC-14, 1916- 1,336,773. Patented Apr. 13, 1920 2 SHEETSSHEET 2.

' Inf/chic)": wi lire'azd ll, 1 14,309

JOHN WILLIAM CALDWELL, OF LEBANON, NEW HAMPSHIRE.

SCREW.

Specification of Letters Patent.

Patented Apr. 13, 1920.

Application filed December 14, 1916. Serial No. 136,867.

To allwhom it may concern:

Be it known that I, JoHN WILLIAM CALD- WELL, a subject of the King of Great Britain, formerly a resident of Coolamon, New South Wales, Australia, and later of the city of Toronto, in the county of York, 1n the Province of Ontario, in the Dominion of Canada, and now residing in Lebanon, in the county of Grafton and State of New Hampshire, U. S. A., have invented certain new and useful Improvements in Screws, of which the following is a specification.

This invention relates more especially to that class of wood screws which are adapted to be used as drive-screws, but my present improvements are also applicable to screws of various styles and sizes, which are to be inserted by turning them into the wood under an endwise pressure, after the usual manner of inserting wood screws by the use of a screw-driver.

A principal object of my present improvements is to furnish a wood screw adapted to be readily and properly inserted either by the driving-in or the turning-in method, and also adapted for use in woods of a wide range of qualities, especially as regards toughness and hardness; other objects and numerous practical advantagesare pointed out in the course of the following description.

The improved screw herein described may be considered as regards some features thereof, as being in the nature of an improvement on the invention which was described and claimed by me in U. S. Patent No. 635,297, dated Oct. 24, 1899, but the present invention relates to features not disclosed in my said prior patent, whereby further and important advantages and modes of action are obtained.

The screw-making industry having become, as is well known, one of the most extensive and important of American manufactures, therefore it is now recognized that a further improvement upon any of the leading kinds of screws, will have a very great utility and value. My present improvements are particularly applicable to woodscrews of the drive-screw kind, which is a branch of said manufactures that has become of great public importance for'the reduction of cost and increasing efliciency of the labor of carpenters and other artisans.

While in its construction my improved wood-screw, is in several respects complex and intricate, comprising various refine- -ments as regards several details and their peculiar combinations and modes of action, yet the screw, as' regards the using thereof, is more readily applied than those heretofore in use. It should be understood that methods and means have been devised,not herein described,-whereby the complex structural features of the screw can be produced for a low cost and with a uniform high quality for the market.

In the accom 'ianying drawing and forming a part of this specification, Figure 1 is a side view or elevation of a wood-screw made in accordance with my present improvements.

Fig. 1* is a diagrammatic view for illustrating the point-portion of the screw as.

seen from the right-hand in Fig. l, and with one of the threads omitted, for more clearly exhibiting the normal relation of the chisellike thread-points, or incising blades, to the tapering point-portion of the screw body.

Fig. 2 is a diagrammatic view corresponding to a longitudinal section in the plane of the axis through the threaded portion of the screw as shown in Fig. 1.

Fig. 2 is a diagramior further illustrating and comparing certain portions of Fig. 2, and is drawn on a much larger scale, as hereinafter explained.

Fig. 2 is a small diagrammatic View supplemental to Figs. 2 and 2*.

Fig. 3 is a plan view of the head of the particular form or style of screw shown in Fig. 1, and is illustrative of a preferred method of making an improved form of slot or nick in the screw head.

Fig. 4 is a sectional side view as seen from below in Fig. 3 and is further illustrative of the construction and preferred mode of manufacture of the screw head.

Fig. 5 is a detail view illustrating further features regarding the use of the screw head shown in Figs. 1, 3 and 4.

Fig. 6 is a detail view illustrative of certain purposes of the aforesafii improvements regarding the head of the screw.

Fig. 7 is a diagrammatic view representing certain portions of Fig. 2 greatly enlarged and with supplementary features illustrative of the action of the screw threads upon the fiber of the wood, as hereinafter more fully explained.

Similar characters designate like parts in all the figures.

'Referring to that in Fig. 1 a screw conical form of head, and an unthreaded body-portion, B, is also provided with the extended core or body portion comprising the cylindrical part D, and the conical part D. The threaded part of the screw may comprise, as herein shown, both of the parts D and D. In; the illustration, Fig. l, the screw is shown having two interspaced threads T, T, which at the point of the screw terminate in broad and chisel-like incising blades, 6, b", respectively. While the duplex threading of the screw will usually be preferred for use on drive screws, a single having an ordinary thread may be used in some cases. In either I case, of course the pitch may be varied to suit the requirements. The illustrations of the successive thread sections along one side of the diagram, Fig. 2, as for instance at the right side of the axial line 0 thereof, corresponds to the thread sections on the right hand side of the screw in Fig. 1 whether the screw shall be provided with the duplex thread, as shown, or with a single thread in lieu thereof. 7

The manner in which the screw threads T, T, terminate in the incising blades 6, b,

is illustrated in Fig. 1*, where the line 00 is drawn tangential to the outer edge of the thread T at a place near the lower and wider end of this thread. Thus the angle 4;, made by the lines a: and n indicates the inclination of the lower surfaces of said blades,--in a screw having the proportions asdraWn -and the considerable angle which is effective for causing the screw to turn when it is properly driven down.

The point-end portion D of the screw body being taperedand preferably tapered in the conical manner shown,-to near a point, as indicated at line a Fig. 2, and the thread on such tapered portion having a height which increases while the thickness (in the part thereof which adjoins said tapered portion of the screw body) decreases from the large end toward the small end of said tapered body portion, results in the screw having a compressive action upon the fiber after this is separated by incising blades, as b, b, (Fig. 1), with which the two duplicate and interspaced threads T, T, are respectively provided; this peculiar action which is not fully shown in Fig. 2, is separately illustrated in the specially arranged diagrammatic view, Fig. 7. In Fig. '7, the lower space d6 represents. the space (l6 near the point of the screw in Figs. 1 and 2, while the upper space d5, Fig. 7, represents the space at (Z5 1n Figs. 1 and 2. When accurately made in the manner described, the thread from the point of said tapered body portion D upwardly toward the larger end thereof at line n3, the distance,measured on and along the line 03,between the the drawing, it will-be seen successive threads will constantly decrease in said upwarddirection, (as already explained), so that du'rin the downward movement of the screw reTatively to a given portion of the wood, this portion is graduall compressed in a direction longitudina ly of the screw as indicated by the distance'd5 between the screw threads t7, t8, in Fig. 7 ,being shorter than the distance d6 between the threads t2, t3. During this same time the entire mass of the wood which is thus compressively acted upon in one direction by the adjacent threads, is pushed back in a transverse direction from the axial line 0 of-the screw by the wed ing action clined line 03) of the said tapered pointend portion D of the screw body. This peculiar compressive action upon the wood, of course, is facilitated and modified by the gradually changing position or angular, rotative shift of the upper profile of the thread which iggdescribed in connection with Fii 2 further and very important object is accomplished by the peculiar construction of the screw especially in that part within the 'zone .24, and between the lines n3 and By making the core portion D of a slim and sharply pointed form as herein illustrated,.the screw when driven into the wood has but slight resistance due to the wedging action of the said point portion until after the screw has been driven to a sufiicient depth for engaging several threads (or turns) in the fiber of the wood. During the early stages of this operation, the extreme point portion of the conical part D offers but a very slight resistance, while the higher portion of the thread is being driven and turned into the wood. This part of the thread, as indicated at t2, being of a thin form and much wider than the height of the thread farther up on the screw,-as at (Z2,the result is to provide a relatively extended surface upon the underside of the thread, which surface furnishes a large area for resisting a direct downward movement ofthe screw when this is struck in the usual manner by a hammer, beginning with light initial blows so that in practice the screw may be safely driven into the wood, and will readily turn under the blow, even when the pitch of the screw is made relatively small. The large amount of lower surface of the thread near the point of the screw not only directly resists the. forward movement in the manner indicated,but as a result of that action tends also to cause the screw to revolve more freely and thus to enter the wood in more perfect manner and with a less destructive action upon the fiber of the wood as an immediate result of the blows of the hammer.

A further feature of the operation of the of the side surface (indicated by the'in-' screw when being driven into the wood is that the large height of the thread at the point of the screw, causes said screw-thread to initially incise the wood fiber to a diameter as great, or nearly as great, as the diameter of the cylindrical part D of the body of the screw, so that during the movement of the screw in entering, the outer portion of the wood (between the lines 03 and 04) the wood fiber which has been previously incised, will be gradually compressed by an amount which in practice is fairly approximated by the difference between the distance (Z2 and the distance (23. As a result of that compression, (as will be readily understood, although not herein shown) the Wood may be said to be con densed laterally of the screw substantially in accordance with the reduction of the height of the screw-thread from the line 024 upwardly to the line n3. In this manner, as it appears from my practical investigations, the operation of the screw is in all respects greatly improved, while the screw is made applicable to a wider range of uses,

and for use in woods having a wide range of qualities.

In the left-hand part of the diagram Fig. 2, the lines which correspond to the said lines 03 and 04, are designated by 03 and 04, respectively, and the extreme diameter of the screw-thread at the line n4 is here represented as being substantially the same as the diameter of the cylindrical (or nearly cylindrical) part D of the screw-body B; these are the preferred proportions, which, however, may be varied to some extent in some cases.

The progressive widening, or increasing height, of the screw thread downwardly through the zone 24,-from the transverse line n3 to the end of the screw-point at the line n4,has the effect of largely increasing the total area of thread-surface as compared with the area of surface contact which otherwise the thread would have when the screw is inserted in the wood. This feature will be evident from a comparison of said thread-sections as illustrated in Fi .3, where the described increase of the l'ieight-of thread is indicated (in the right-hand part of said diagram) by the divergence of the outer threadbounding line 04 from the inner threadbounding line 03, which is one side line of said tapered portion D. The lower face, or surface, of the thread should have a uni form pitch throughout and along the length of said tapered portion D, and this taken with the described increasing height and decreasing thickness of the thread, will result in the upper side of the thread,in that part thereof which adjoins the body portion D'having a pitch reater than the pitch of said lower surface. It is also at the inner shown. in Fig. 2, how the two interspaced threads each have such an increase of height as explained, that at the point-end of the screw, the outside diameter of the threads corresponds to, and is as great as, the inside diameter at the larger end of said tapered body-portion D.

A leading feature and object of my invention is to furnish a screw having a thread which, while it ma be of uniform pitch, may have a decreasing height, combined with a decreasing width of the threadspace, or core-surface in passing from the point, as at line n4, upwardly to the line n3,see Fig. 2. This feature is indicated by the distance d6 being relatively muchreater than the distance d5, while the eight of thread at (Z3 is, similarly, in excess of the height at ($2. This decreasing width of thread-space from d6 to $5 corresponds to the increasing width of the zone or area of integral connection of the thread to the tapered core-portion of the screw body, which distances or widths are designated by 58 and (Z7, respectively. Thus, as clearly set forth by the lines of the drawing in Fig. 2, the decreasing diameter of the core D from line 013 to line n4, is accompanied by a-decreasing widthof the base of the thread-section from the distance or amount d7 to the amount J8, and also by a corresponding increase of the height of the thread from the height d2 to the height 0Z3, so that the wood-engaging surface of the thread increases with each turn of thread from the line n3 to the line a4, and is accompanied by a corresponding increase in the sharpness of the outer, or cutting edge of the thread; these several progressive features are also obtained in connection with and by means of a further improvement, as follows:-

Referring to the enlarged diagram, Fig. 2, and on comparing the same with Fig. 2, it will be seen that the transverse lower profile of the thread (as seen in a sectional view thereof) is a curved line, t2, which has the same position or angular direction end thereof relatively to the side-line c3 of the core-length D, throughout the length 24, so that the said lower profile has the same contour throughout the length thereof, at all points in the zone 2-4, but is of a constantly increasing length outwardly from the line 03, from the line 023 downwardly to the line n4; thus the lower" profile may be said to have,in the form shoWn,-a constant position and contour throughout the length of the core-portion D; this is indicated by the uniformity of the angles y and 3 in Figs. 2 and 2, respectively.

Along the side line 03 of the conically tapered part D, the several pairs of threadsection profiles are made up of a lower proper profile Fig. 2, with on the circle t8", and in each of said pairs the lower profiles have, preferably, (as shown) a uniform angular position or relation at the inner end thereof to the side line 03, while the upper profiles have a similarly uniform angular relation to the axial line 0; also, the radiu of said circle t8 of the lower profile is in excess of the radius of the circle t8 of the upper profile, for thereby giving-to the series of thread-sections a preferable ratio of increasing width of that part of the thread which directly adjoins'said tapered point-portion D of the screw body. The 'effect,--in the resulting thread-section proportions,-of this system of screw-thread construction, is to give to those sections an acute or sharp form at and near the screw point, (at or near line a4), and a more obtuse form in the larger part of the screw, as near line a3. Thus the thread is of a thin and sharp proportion near the point for securing a highly efiective incising action, and farther up on the part D is proportioned for a relatively greater holding power with a less acute angular relation of said upper and lower proles at their point of intersection, at the outer edge of the thread.

Said profile circles t8 and $8 of Fig. 2 are, of course, the same as indicated in relation to the particular thread-section 68, where they coincide, respectively, with the lower and the upper pro- T he said circle, or circular arc 258 of the lower profile is drawn in Fig. 2 with the center point 78 thereof on the axial line 0, while the arc t8 of the upper profile has the center point r8 thereof on the side line, or cone-line, as 03; this relation of said arcs and center'points is herein indicated as being continued throughout the length of the screw thread. Ubviously said lines 0 and c3 intersect at or near the point of the screw,-usually somewhat below the line n4, as at point 202, Fig. 2. And the size and area of the section of the thread at 62, will be chiefly determined by the excess of the radius line s over the radius line s.',- this gives to the section 252 not only the desired thin and knife-like character, but also gives to that section'a proper and effective, though relatively small, inclination downwardly and outwardly from the screw body, so that, on the screw being driven in, the under surface of the thread tends somewhat to draw the wood fiber toward the center, and thus to counteract to some extent the earlier or initial wedgingapart action of the point of part D upon the wood. Thus, it may be said, that in the plane of axis 0, the two opposite thread- .sections t2, t2 have,taken together and when the profiles are curved,a cup-shaped formation, and this is shown continued rest We throughout the length of the thread; but an analogous cup-shaped formation may be secured when the thread-section profiles are modified, as for instance, in Fig.- 2 where straight-line profiles are shown.

For more fully illustrating the profile and thread-section features and cooperative relations, scribed, the thread-sections at t2 and t8, Fig. 2, are shown superposed,-on. an enlarged scale,-in Fig. 2 in which the profile circles t2 and t2 are those of the lower and upper profiles respectively of section t2. The circle 158 indicates the same profile line as 62 when shifted, or rotated, to a different position relatively to the side-line 03. This apparent shifting, however, of the profile line $2 is herein shown accomplished by taking for this particular profile,at each of the respective thread-sections from 62 to i8,a different part of the same profile-circle which is shown at .28 in Fig. 2*. By comparing said series of sections from 252 to 68, as illustrated in Figs. 2 and 2, it will be seen that the position of the several sections, relatively to the screw axis 0 and as indicated by a line, tion, continuously changes, being at a minimum angular position at $2, and at a much greater angle at t8. For convenience, these lines are shown as straight lines, being designated by $2 for the section 62, and by 258 for-the section t8, so that the angle 32 Fig. 2", may be said to represent the aforesaid apparent relative shift of the said upper profile from position 2 to position $8. In the modification, Fig. 2 the several corresponding lines and angles are designated by the same characters, respectively, as in Fig. 2 but with the addition of doubleprime marks.

From the foregoing explanation as illus trated, it will now be evident that my improved wood screw, in the more complete form and construction thereof, may be said to have a thread which near the point is of an incising and fiber-separating character; that the thread sections, in the plane of the screw axis, and from the point toward the head, have a continuously decreasing height with a continuously increasing width or base, while the apex of the section,-which is at the intersection of the profile-lines, as 252, 252 (Fig. 2),is a thin or knife-like edge which may extend the whole length of the thread; and, that the threadsections or functions, whichare above de-,

as 62, through the sechave,-or may have, as illustrated,a contures of these lines,which are thecurvature of the thread profiles,may be uniform throughout the length of the thread, this beingthe construction shown in Figs. 1, 2 and2. Also, it will be apparent that the said features of construction may be employed in screws having a single thread, (not herein shown), or in those having two inte'rspaced threads, as T,- T, in Fig. 1.

One part of my invention relates to a screw having in the body or core thereof, the tapered point-end portion D,preferably conically tapered,combined with a substantially cylindrical part, as D, andhaving a screw thread which is of a decreasing height from the small end of the part to the part D, said thread being continued on the part D in a relatively uniform size and shape of section. By these means, that part of the screw from line we to line n3, (see Fig. 2) fully effects the desired separation and condensing of the wood fiber, so that the wood directly acted upon by the thread is thus prepared in a gradual and progressive manner and without substantial injury, into a condition and shape which may be described as a wellformed nutthread, into which the relatively uniform thread of the part D, will follow without a further disturbance or rearrangement to any material extent of the fiber in that portion of the wood. Thus the threads on the tapered part B constitute, in a proper sense, a nut-forming instrumentality, whereby to preliminarily shape a thread-engaging portion of the wood for properly receiving the thread of the non-tapered part B of the screw. These features and results, especially in relation to cabinet-work, are ob viously of very great practical importance.

Another part of my invention relates to the construction of the tapered part D, especially when such taper is a cone,an0l to the proportions thereof relatively to a geometric cone which includes within itthe threads on said part I). in Fig. 2 the side lines 03, c3. of the cone D are shown intersecting, or meeting, at the point'p2 on the axis 0, which point (near to the line n4) thus becomes the apex of the cone ofthe part D. The lines c4, 04:, are, similarly, the side lines of a geometric cone, which I designate as the thread-cone and which has its apex at 373 in said axial line 0. These two thread-bounding cones are so proportioned, in the preferred construction, that the distance between said apexes p2, p3, approximates to the length of said cone D. Said lengths or distances, in practice, though preferably equal may vary within a ratio of from about three-fourths to about one and one-half times the length of the cone D. Vlhen made within these proportions, (but always with the distance (Z3 greater than 012) and with the taper of the part D of conical form, and with the thread-including geometric figure also of the described conical form, the changing form of the thread from the line 114 to the line n3 is found to be particularly effective not only in entering the wood by the driving or by the turning of the screw, but also for the forming of the said nut thread. in the wood.

By reason of the wide spread of the threads at t2, t2, as compared with the size, (and, hence the small resistance oflered by) the point portion of the part D, the screw normally begins to turn instantly when the driving-in operation is begun, whereas in the case of ordinary drive-screws the screw only begins to turn while under the pressure of the hammer blow, after the screw has entered the wood a considerable distance, thereby V crushing and disintegrating the very same fiber which, later on, is required for holding the screw in place. By means, however, of my present improvements, the wood fiber is more gently separated as by rotating chisel-points, and is thus first separated and then gradually pressed together and condensed into a superior holding means, thereby effectively overcoming a most serious objection hitherto found to the general use of drive-screws in the better kinds of house finishing, and in cabinet work. Thus by means of my present invention, the range of uses to which drive screws may be successfully applied, is greatly extended, while the holding power of the screws is correspondingly improved.

When the screw is driven in,especially if the wood is only moderately resistant, as pine or the like,the wood fiber tends somewhat at first to yield and subside under the force and impactthereon of the lower thread-surface, and this together with the turning of the screw, naturally causes the thread,-at any given point or location in the woocL-to move into the mass of woodfiber in about the direction of'the line t8 of Fig. 2 and in this movement to compensate for any such subsidence by the gradual widening of the base-line of the thread-section triangle at that-given point by and during the progress of the thread past such point or location. Thus one of the injurious effects commonly found in the use of ordinary drive-screws, is corrected and overcome.

On account of the necessity, in practice,-

for driving the screw a considerable distance into the wood by means of the hammer, it is particularly desirable in connection with my present improvements relating to the thread ed part of the screw, also to provide in combination therewith an improved head portion whereby to resist the deformation of the head by hammer blows which are heavy enough to cause the turning of the screw to take place in a proper manner. For meeting these requirements,-especially in said screw the head as trated in the sectional views, Figs. 4c and 5.

Thus in the preferred form herein illustrated (see Fig. i), the bottom lines, he, k5, of the slots M, 71.5, are arcs of circles which intersect at a point, as k6, outside,

or above, the face-line or top face of the head. Thus the usual continuous nick or slot-acr6ss the head of the screw is interrupted by the transverse portion b6 and consists of said two separate slots or spaces ha, 715, arranged in alinement. A convenient manner of making these slots is indicated in Figs. 3 and 4:, where the two milling cutters 4, 4, are carried, byspindles indicated at 5, 5', which may be operated in some suitablamachine or-milling device of a well known character, but not herein shown. By means of a milling apparatus'such 'as here clearly indicated, said slots h4,h5, may be simultaneously milled out from the usual solid form of'blank screw-head. When the screws are thus specially slotted they may be turned by means of a screw driver, as B, having a form as indicated in Fig. 5. The

large area of metal thus left at k6 in the middle part of the head, so strongly connects together said side portions thereof, that notwithstanding the frictional resistance of the screw ,threads, the screw may be safely struck by the hammer, with a minimum of tendency to the deformation of the head. Thus there is a direct coaction of the described peculiar construction of the screw-' point (with its progressively increasing height of thread) with the head of the screw, this head being specially arranged for resisting the blows necessary for revolving by means of said thread when the screw is driven into the wood; this is particularly important when wood is of a firm quality.

In practice, as is well known, a slight misdirection of the blow of the hammer as K, Fig. 6, when directed against one side of the head of an ordinary screw head, tends to bend over that side, as, at 72.9. This is because the usual slot, as w, so greatly divides the head into two parts that one said part is easily deflected or crushed out of proper position. For more fully illustrating this tendency, the hammer K in Fig. 6 is shown tipped over to an excessive angle so that said side k9 is thereby excessively deformed; but the deformation here illustrated is very injurious, even when taking place to a small extent.

Having thus described my invention, I

- claim:

imam

and having on such tapered portion a thread which increases in height from the large end toward the small end of said tapered point-end portion, and which at the point of the screw is-provided with an incising blade.

2. A screw ha. a point-end portion of the body thereof tapered to near a point, and having on'such tapered pertiona pair of interspaced threads, the height of each saidthread increasing from the large end toward the small end of said tapered portion, and at said point having each thread terminating in an incising blade.

3. A screw having the point-end portion of the body thereof tapered to near a point, :and having on such tapered portion a thread the height of which increases and the thickness thereof decreases from ,the large end toward the point end of said tapered body portion, and which at said point is provided with an incising blade. v

4. A screw having the point-end portion of the body thereof tapered to near a point,

and having on such tapered portion athread the height of which continuously increases and the thickness thereof continuously decreases from the large end toward the point end of said tapered body portion.

5. A screw having the point-end portion of the body thereof conically tapered to the point thereof, and having on such tapered portion a thread the height of which continuously increases and the thickness thereof continuously decreases from the large end to the point of, and throughout the entire length of said tapered body portion.

6. A screw having the point-end portion of the body thereof conically tapered to near a point, and having on such tapered portion a pair of interspaced threads the height of each of which continuously increases and the thickness decreases from the large end toward the point end of said tapered body portion, and which at said point is provided with an incising blade.

7 A screw having the point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread the height of which increases and the thickness thereof continuously decreases from the large end to'the point of said tapered bodyof the body thereof tapered to near a point,

and having on such tapered portion a thread which in section is three-sided and has the curvilinear profiles and the height of which increases and the thickness thereof decreases from the large end to the point of said tapered body portion, and there terminates in an incising blade.

10. A screw having the point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread which in section is three-sided and has two curvilinear profiles and the height of which continuously and uniformly increases and the thickness thereof continuously decreases from the large end to the small end of said tapered portion.

11. A screw having the point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread which in section has curvilinear profiles, and the height of which continuously and uniformly increases and the thickness thereof continuously and uniformly decreases from the large end to the small end of said tapered portion, and there terminates in an incising blade.

12. A screw having the point-end portion of the body thereof conically tapered to near a point, and having on such tapered portion a thread which in section has a curvilinear upper-profile, and the height of which continuously and uniformly increases and he thickness thereof continuously and uniformly decreases from the large end to the small end of said conically tapered portion, and there terminates in an incising blade.

13. A screw having the point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread which in section has a concave lower profile and a convex upper-profile, and the height of which continuously increases and the thickness thereof continuously decreases from the large end toward the point-end of said tapered body portion.

14. A screw having the point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread which in section has a concave lower profile and a convex upper-profile, and the height of which continuously and uniformly increases and the thickness thereof continuously decreases from the large end to the small end of said tapered portion, and there terminates in incising blades.

15. A screw having the point-end portion of the body thereof conically tapered to near a point, and having on such tapered portion a thread which in section has a concave lower profile and a convex-upper-profile, and the height of which continuously and uniformly increases and the thickness thereof continuously decreases from the large end to the small end of said conically tapered portion.

16. A screw having the point-end portion of the body thereof conically tapered to near a point, and having on such tapered portion a thread which in section has a convex upper-profile, and the height of which continuously and uniformly increases and the thickness thereof continuously and uniformly decreases from the large end to the small end of said conically tapered portion, and there terminates in an incising blade.

17. A screw having a point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread which in section is three-sided, and the height of which continuously increases from the large end to the small end of said tapered portion, and-which also in the reverse direc tion has an increasing inclination down wardly and outwardly.

18. A screw having a point-end portion of the body thereof tapered to near a point, andv having on such tapered portion a thread which in section is threesided with the upper side thereof a convex profile, and the height of which continuously increases from the large end to the small end of said tapered portion, and which also in the reverse direction has an increasing inclination downwardly and outwardly, and at the point terminates in an incising blade.

19. A screw having point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread which in section is three-sided with the lower side thereof a concave profile and the upper side thereof convex profile, and the height of which continuously increases from the large end to the small end of said tapered portion, and which also, in the reverse direction has an increasing inclination downwardly and outwardly.

-2O. A screw having a point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread which in section is an acute triangle with the apex thereof at the outer edge of the thread, and which continuously increases in height and also in acuteness of said section angle from the large end to the small end of said tapered portion.

21. A screw having a point-end portion of the body thereof tapered to near a point. and having on such tapered portion a thread which in section is an acute triangle with curvilinear sides and with the apex thereof at the outer edge of the thread, and which continuously increases in height and also in acu. :ness of said section angle from the large end to the small end of said tapered portion.

22. A screw having the point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread the height of which increases from the large end toward the small end of said tapered body portion, said thread on'the inner part pitch greater than the pitch of the lower side thereof, and having an incising blade at the point-end of the screw.

2% A screw having the point-end portion of the body thereof tapered to near a point, and having on such tapered portion a pair of interspaced threads the height of each of which increases from the large end toward the small end of said tapered body portion, each of said threads on the inner part thereof which adjoins the surface of said tapered portion, having on the upper side a pitch greater than the pitch of the lower side, and each thread terminating in an incising blade.

25. A screw having a point-end portion of the body thereof tapered tone-ar a point, and

having on such tapered portion a thread the height of which increases from the large end to the small end of said tapered body por tion, the screw-thread at the point of the screw having an outside diameter as great as the inside diameter thereof at the large end of said tapered point-end portion of the screw body, and said thread terminating at the screw point in an incising blade.

26. A screw having a point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread the height of which increases and the thickness decreases from the large end toward the small end of said tapered bodyportion, the screw-thread at the point of the screw having an outside diameter as great as the inside diameter thereof at the large end of said tapered point-end portion of the screw body.

27. A screw havlng a point-end portion of the body thereof conically tapered to near-a point, and having on such tapered portion a thread the height of which continuously increases and the thickness thereof decreases from the large end to the small end of said tapered body portion, the screw-thread at the point of the screw having an outside diameter as great as the inside diameter thereof at the large end of said tapered point-end portion of the screw body, and said thread terminating at the screw point in an incising blade.

28. A screw having the point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread the height of which continuously inmamas creases from the large end toward the small end of said conically tapered portion, said thread in the part thereof w ich adjoins the surface of said tapered portion, having on the upper side thereof a pitch greater than the pitch of the lower side thereof.

29. A screw having the point-end portion of the body thereof conically tapered to near a point, and having on such tapered porthan the pitch of the lower side thereof,

and the screw-threads at the point of the screw having an outside diameter as great as the inside diameter thereof at the large end of said tapered point-end portion of the screw body.

30. A screw having the point-end po'r= tion'of the body thereof conically tapered to near a point, and having on such tapered portion a thread which in section has convex upper profiles and the height of which increases from the large end to the small end of said conically tapered portion, said thread adjoining the surface of said tapered portion, having on the upper side thereof a pitch greater than the pitch of the lower side thereof, and the screw-thread at the point of the screw having an outside diameter as great as the inside diameter thereof at the large end of said tapered point-end portion of the screw body, and said thread terminating at the screw point in an inci'sing blade.

31. A' screw having thepoint-end portion of the body thereof conically tapered to near a point and having on such tapered portion a pair of threads each of which in section has a concave lower profile and a convex upper profile, and the height of each' of which increases'from the large end toward the small end of said conically tapered portion, said threads adjoining the surface of said tapered portion, having on the upper side thereof a pitch greater than the pitch of the lower side thereof, and the outside diameter of the screw-threads at the point of the screw corresponding with the inside diameter thereof at the large end of said tapered point-end portion of the screw body.

32. A screw having a point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread which in section has an underside profile with a relatively uniform angular relation to the surface of said tapered portion, and also having a conveXly-curved upper profile with a continuously increasing angular relation to said surface, from the small end toward the large end of said point-end portion.

33. A screw having a point-end portion of the body thereof tapered to near a point, and having on such tapered portion a thread which in section has an underside curved" profile with a relatively uniform curvature and angular relation to the surface of said tapered portion throughout the length thereof, and also having a convexly-curved upper profile with a uniform curvature throughout the length thereof and with a continuously increasing angular relation to said surface, from the small end toward the large end of said point-end portion.

34. A screw having a point-end portion of the body thereof tapered to near a point,

and having on such tapered portion a thread which in section has an underside curved profile with a relatively uniform curvature and angular relation to the surface of said tapered portion throughout the length thereof, and also having a convexly-curved upper profile with a uniform curvature throughout the length thereof and with a continuously increasing angular relation to said surface, from the small end toward the large end of'said point-end portion, and the thread having said upper profile. of a curvature greater than the curvature of said underside profile.

JOHN WILLIAM CALDWELL.

Witnesses:

CLARENCE E. HrBBARo, ERMA M. GATES. 

